core/lib/Drupal/Core/Cache/Cache.php               |   28 ---------------
 core/lib/Drupal/Core/Cache/DatabaseBackend.php     |   36 +++++++++++++++++---
 core/lib/Drupal/Core/Cache/MemoryBackend.php       |   34 ++++++++++++++++--
 .../Core/EventSubscriber/HtmlViewSubscriber.php    |   26 +++++++++++++-
 .../system/Tests/Bootstrap/PageCacheTest.php       |    2 +-
 5 files changed, 89 insertions(+), 37 deletions(-)

diff --git a/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
index feb3c77..c2f5fe6 100644
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -70,32 +70,4 @@ public static function getBins() {
     return $bins;
   }
 
-  /**
-   * 'Flattens' a tags array into an array of strings.
-   *
-   * @param array $tags
-   *   Associative array of tags to flatten.
-   *
-   * @return array
-   *   An indexed array of flattened tag identifiers.
-   */
-  public static function flattenTags(array $tags) {
-    if (isset($tags[0])) {
-      return $tags;
-    }
-
-    $flat_tags = array();
-    foreach ($tags as $namespace => $values) {
-      if (is_array($values)) {
-        foreach ($values as $value) {
-          $flat_tags[] = "$namespace:$value";
-        }
-      }
-      else {
-        $flat_tags[] = "$namespace:$values";
-      }
-    }
-    return $flat_tags;
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index ad873e6..4f98c50 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -163,7 +163,7 @@ public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array
    * Actually set the cache.
    */
   protected function doSet($cid, $data, $expire, $tags) {
-    $flat_tags = Cache::flattenTags($tags);
+    $flat_tags = $this->flattenTags($tags);
     $deleted_tags = &drupal_static('Drupal\Core\Cache\DatabaseBackend::deletedTags', array());
     $invalidated_tags = &drupal_static('Drupal\Core\Cache\DatabaseBackend::invalidatedTags', array());
     // Remove tags that were already deleted or invalidated during this request
@@ -237,7 +237,7 @@ public function deleteMultiple(array $cids) {
   public function deleteTags(array $tags) {
     $tag_cache = &drupal_static('Drupal\Core\Cache\CacheBackendInterface::tagCache', array());
     $deleted_tags = &drupal_static('Drupal\Core\Cache\DatabaseBackend::deletedTags', array());
-    foreach (Cache::flattenTags($tags) as $tag) {
+    foreach ($this->flattenTags($tags) as $tag) {
       // Only delete tags once per request unless they are written again.
       if (isset($deleted_tags[$tag])) {
         continue;
@@ -307,7 +307,7 @@ public function invalidateTags(array $tags) {
     try {
       $tag_cache = &drupal_static('Drupal\Core\Cache\CacheBackendInterface::tagCache', array());
       $invalidated_tags = &drupal_static('Drupal\Core\Cache\DatabaseBackend::invalidatedTags', array());
-      foreach (Cache::flattenTags($tags) as $tag) {
+      foreach ($this->flattenTags($tags) as $tag) {
         // Only invalidate tags once per request unless they are written again.
         if (isset($invalidated_tags[$tag])) {
           continue;
@@ -358,6 +358,34 @@ public function garbageCollection() {
   }
 
   /**
+   * 'Flattens' a tags array into an array of strings.
+   *
+   * @param array $tags
+   *   Associative array of tags to flatten.
+   *
+   * @return array
+   *   An indexed array of flattened tag identifiers.
+   */
+  protected function flattenTags(array $tags) {
+    if (isset($tags[0])) {
+      return $tags;
+    }
+
+    $flat_tags = array();
+    foreach ($tags as $namespace => $values) {
+      if (is_array($values)) {
+        foreach ($values as $value) {
+          $flat_tags[] = "$namespace:$value";
+        }
+      }
+      else {
+        $flat_tags[] = "$namespace:$values";
+      }
+    }
+    return $flat_tags;
+  }
+
+  /**
    * Returns the sum total of validations for a given set of tags.
    *
    * @param array $tags
@@ -366,7 +394,7 @@ public function garbageCollection() {
    * @return int
    *   Sum of all invalidations.
    *
-   * @see \Drupal\Core\Cache\Cache::flattenTags()
+   * @see \Drupal\Core\Cache\DatabaseBackend::flattenTags()
    */
   protected function checksumTags($flat_tags) {
     $tag_cache = &drupal_static('Drupal\Core\Cache\CacheBackendInterface::tagCache', array());
diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index c571a5e..a7faca6 100644
--- a/core/lib/Drupal/Core/Cache/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Cache/MemoryBackend.php
@@ -101,7 +101,7 @@ public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array
       'data' => $data,
       'created' => REQUEST_TIME,
       'expire' => $expire,
-      'tags' => Cache::flattenTags($tags),
+      'tags' => $this->flattenTags($tags),
     );
   }
 
@@ -123,7 +123,7 @@ public function deleteMultiple(array $cids) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::deleteTags().
    */
   public function deleteTags(array $tags) {
-    $flat_tags = Cache::flattenTags($tags);
+    $flat_tags = $this->flattenTags($tags);
     foreach ($this->cache as $cid => $item) {
       if (array_intersect($flat_tags, $item->tags)) {
         unset($this->cache[$cid]);
@@ -160,7 +160,7 @@ public function invalidateMultiple(array $cids) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::invalidateTags().
    */
   public function invalidateTags(array $tags) {
-    $flat_tags = Cache::flattenTags($tags);
+    $flat_tags = $this->flattenTags($tags);
     foreach ($this->cache as $cid => $item) {
       if (array_intersect($flat_tags, $item->tags)) {
         $this->cache[$cid]->expire = REQUEST_TIME - 1;
@@ -178,6 +178,34 @@ public function invalidateAll() {
   }
 
   /**
+   * 'Flattens' a tags array into an array of strings.
+   *
+   * @param array $tags
+   *   Associative array of tags to flatten.
+   *
+   * @return array
+   *   An indexed array of strings.
+   */
+  protected function flattenTags(array $tags) {
+    if (isset($tags[0])) {
+      return $tags;
+    }
+
+    $flat_tags = array();
+    foreach ($tags as $namespace => $values) {
+      if (is_array($values)) {
+        foreach ($values as $value) {
+          $flat_tags["$namespace:$value"] = "$namespace:$value";
+        }
+      }
+      else {
+        $flat_tags["$namespace:$values"] = "$namespace:$values";
+      }
+    }
+    return $flat_tags;
+  }
+
+  /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::isEmpty().
    */
   public function isEmpty() {
diff --git a/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
index 5bcaabf..c71d86a 100644
--- a/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
@@ -77,7 +77,7 @@ public function onHtmlPage(GetResponseForControllerResultEvent $event) {
       // recommended.
       $response = new Response((string) $this->pageRenderer->render($page), $page->getStatusCode());
       if ($tags = $page->getCacheTags()) {
-        $response->headers->set('X-Drupal-Cache-Tags', implode(' ', \Drupal\Core\Cache\Cache::flattenTags($tags)));
+        $response->headers->set('X-Drupal-Cache-Tags', $this->flattenCacheTags($tags));
       }
       if ($keys = $page->getCacheKeys()) {
         $response->headers->set('cache_keys', serialize($keys));
@@ -105,4 +105,28 @@ static function getSubscribedEvents() {
     return $events;
   }
 
+  /**
+   * 'Flattens' a tags array into an array of strings.
+   *
+   * @param array $tags
+   *   Associative array of cache tags to flatten.
+   *
+   * @return string
+   *   A space-separated liste of flattened cache tag identifiers.
+   */
+  protected function flattenCacheTags(array $tags) {
+    $flat_tags = array();
+    foreach ($tags as $namespace => $values) {
+      if (is_array($values)) {
+        foreach ($values as $value) {
+          $flat_tags[] = "$namespace:$value";
+        }
+      }
+      else {
+        $flat_tags[] = "$namespace:$values";
+      }
+    }
+    return implode(' ', $flat_tags);
+  }
+
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
index 45a94e9..056a908 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -66,7 +66,7 @@ function testPageCacheTags() {
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('page')->get($cid);
     sort($cache_entry->tags);
-    $this->assertIdentical($cache_entry->tags, array('content:1', 'system_test_cache_tags_page:1', 'pre_render:1'));
+    $this->assertIdentical($cache_entry->tags, array('content:1', 'pre_render:1', 'system_test_cache_tags_page:1'));
 
     Cache::invalidateTags($tags);
     $this->drupalGet($path);